The focus of our laboratory has been the recognition of a cardiac progenitor cell (PC) that forms substantial quantities of cardiomyocytes after infarction. Although this work has been successful, to prevent ischemic myocardial damage acutely and the development of an ischemic myopathy chronically, we need to identify a PC capable of restoring the integrity of injured coronary vessels and/or creating de novo conductive coronary arteries and their distal branches. Theoretically, in a manner comparable to hematopoietic stem cells that can repopulate and completely reconstitute the ablated bone marrow, PCs may possess the inherent ability to rebuild the damaged myocardium and convert a severely diseased heart into a physiologically functional heart. Based on this premise, this renewal application will test whether resident PCs conform to a non-uniform cell pool which sustains myocardial homeostasis through the activation and commitment of distinct cell classes devoted respectively to the replacement of smooth muscle cells (SMCs) and endothelial cells (ECs) in coronary vessels and cardiomyocytes. The hypothesis to be tested is that vascular niches are present within the wall of the coronary circulation and vascular progenitor cells (VPCs) are stored within vascular niches. VPCs are viewed as a subset of the PC pool which possesses specialized functions predominantly devoted to the turnover of SMCs and ECs and vasculogenesis. For this purpose, we need profound understanding of the biology of cardiac PCs and must determine whether this PC pool includes subsets which have powerful vasculogenic properties. Identification of a coronary VPC able to differentiate predominantly into SMCs and ECs would suggest that the heart possesses the inherent ability to create the various portions of the coronary circulation. However, a critical issue concerns the origin of VPCs and the mechanisms involved in the preservation of the VPC compartment in the coronary circulation. This may be accomplished by migration of primitive cells from the bone marrow to the vessel wall, asymmetric and symmetric division of resident VPCs within the vascular niches or both. Therefore, the long-term objective of this proposal is the acquisition of fundamental knowledge on the function of VPCs and to determine whether niche homeostasis is regulated by extrinsic and/or intrinsic cellular processes. The identity of VPCs and the mechanisms involved in the activation and differentiation of this PC class into SMCs and ECs will be evaluated by a combination of cellular and molecular approaches together with in vivo experimentation to document its efficacy in the restoration of the integrity of the coronary vasculature and its regeneration. Ultimately, VPC may be employed to replace damaged large coronary arteries with newly formed vessels and to correct rarefaction of resistance coronary arterioles and capillary structures by expansion of the cardiac microcirculation. If this were possible, cell therapy may be employed to interfere with ischemic injury, the prevailing cause of human heart failure and prevention may supersede the need for myocardial regeneration.